This invention relates to a rotational position detection device utilizing reluctance change and, more particularly, to a phase-shift type rotational position detection device detecting reluctance change in the form of change in an electrical phase angle of an output ac signal.
As a rotational position detection device utilizing reluctance change, there i s a we 11 known rotary type differential transformer called Microsin. This transformer which converts a rotational position to a voltage level has the disadvantage that it tends produce an error due to disturbance. In this transformer, there are disadvantages that, for example resistance of the coil changes due to change in the temperature with the result that the level of a detection signal change, the amount of attenuation in the level in a signal path from the detector to a circuit utilizing its detection signal differs depending upon the distance of signal transmission, and the variation in the level due to noise appears directly as a detection error.
The applicant has previously proposed a phase-shift type rotational position detection device capable of accurately detecting a rotational position without being affected by the variation in the output level due to disturbance or other causes (U.S. Pat. Nos. 4,604,575, 4,612,503, 4,754,220, Japanese Laid-open Patent Publication Nos. sho 57-60212, sho 57-88317, Japanese Patent Publication No. sho 62-58445 and others).
FIGS. 7a-8b schematically show the rotational position detection device previously proposed by the applicant. FIGS. 7a and 8a are front views of the rotational position detection device as viewed from the direction of the axis of rotation and FIGS. 7b and 8b are side views, partly in section, of the same device as viewed from the direction normal to the axis of rotation.
The rotational position detection device shown in FIGS. 7a and 7b includes a stator 71a having plural poles A-D projecting in the direction normal to the axis of rotation (i.e., in the direction of a normal line with respect to the axis of rotation) and being disposed at a predetermined interval (e.g., 90 degrees) in the circumferential direction, and a rotor 71b which is inserted in the space defined by the poles A-D of the stator 71a. In other words, the stator 71a is provided opposite to the outer peripheral surface of the rotor 71b.
The rotor 71b is made in a form and of a material which will change reluctance in each of the poles A-D, for example, a column which is eccentric to the axis of rotation. Primary coils 1A-1D and secondary coils 2A-2D are respectively wound about the poles A-D of the stator 71a. Coils are wound in such a manner that the first pair of the poles A and C and the second pair of the poles B and D respectively opposing each other pole in the pair across the rotor 71b act differentially and a differential reluctance change thereby is produced.
The primary coils 1A and 1C wound on the poles (A, C) of the first pair are excited by a sine wave signal ia=I sin .omega.t and the primary coils 1B and 1C wound on the poles (B, D) of the second pair are excited by a cosine wave signal ib=I cos .omega.t. As a result, a composite signal Y of these signals is obtained from the secondary coils 2A-2D. This composite signal Y is a signal Y=sin (.omega.t-.theta.) which is phase shifted by an electrical phase angle corresponding to a rotation angle .theta. of the rotor 71b with respect to a primary ac signal (i.e., an exciting signal of the primary coils) a=I sin .omega.t which constitutes a reference signal.
On the other hand, the rotational position detection device in FIGS. 8a and 8b includes plural poles A-D projecting in a direction parallel to the axis of rotation and disposed at a predetermined interval (e.g., 90 degrees) in the circumferential direction, a stator 81a having a pole E projecting at the axis of rotation, and a rotor 81b having a disc to one side of which the poles A-E oppose.
The rotor 81b is made in a form and of a material which will change reluctance of the poles A-D, for example, a disc which is eccentric to the axis of rotation. Primary coils 1A-1D are wound on the poles A-D of the stator 81a and a secondary coil 2E is wound on the pole E. The first pair of poles A and C and the second pair of poles B and D respectively opposing each other pole in the pair across the pole 2E are wound in such a manner that they act differentially and a differential reluctance change thereby is produced.
The primary coils 1A and 1C wound on the poles (A, C) of the first pair are excited by a sine wave signal ia=I sin .omega.t in the same manner as in the device of FIG. 7 and the primary coils 1B and ID wound on the poles (B, D) of the second pair are excited by a cosine wave signal ib=I cos .omega.t. As a result, a composite signal Y is obtained from the secondary coil E. This composite signal Y is a signal Y=sin (.omega.t-.theta.) which is phase shifted by an electrical phase angle corresponding to a primary ac signal (i.e., an exciting signal of the rotor 81b) ia=I sin .omega.t or ib=I cos .omega.t) which constitutes a reference signal.
Since the rotational position detection device is an auxiliary part for controlling the operation of a machine tool and other industrial machines, a smaller type of rotational position detection device is desirable in respect of the area and capacity for installing it.
In the case of the rotational position detection device of FIGS. 7a and 7b, thickness of the stator 71a and the rotor 71b, i.e., the size in the direction of the axis of rotation, cannot be reduced because the surfaces of the coils face the axis of rotation. Since the stator 71a is provided in a manner to enclose the rotor 71b, reduction in the size of the rotational position detection device in the diametrical direction as well as in the direction of the axis of rotation is limited by the size and length in the direction of diameters of the rotor 71b and the stator 71a.
On the other hand, in the case of the rotational position detection device of FIGS. 8a and 8b, the stator 81a opposes to one surface of the rotor 81b and, therefore, the size of the device in the direction of the diameter can be reduced as compared with the device of FIGS. 7a and 7b by providing the stator 81a within the diameter of rotation of the rotor 81b. Since, however, the poles A-E of the stator 81a are projecting, there is a limit to reducing thickness of the device in the direction of the axis of rotation as compared with the device of FIGS. 7a and 7b.